Rename all?, any? and empty? (#6464)

Rename `all?`, `any?` and `empty?` to `all`, `any` and `is-empty` for sake of simplicity and consistency.

- More understandable for newcomers, that these commands are no special to others.
- `?` syntax did not really aprove readability. For me it made it worse.
- We can reserve `?` syntax for any other nushell feature.

tests/shell/pipeline/commands/internal.rs

@@ -191,7 +191,7 @@ fn run_custom_command_with_flag() {
     let actual = nu!(
         cwd: ".",
         r#"
-        def foo [--bar:number] { if ($bar | empty?) { echo "empty" } else { echo $bar } }; foo --bar 10
+        def foo [--bar:number] { if ($bar | is-empty) { echo "empty" } else { echo $bar } }; foo --bar 10
         "#
     );
 
@@ -203,7 +203,7 @@ fn run_custom_command_with_flag_missing() {
     let actual = nu!(
         cwd: ".",
         r#"
-        def foo [--bar:number] { if ($bar | empty?) { echo "empty" } else { echo $bar } }; foo
+        def foo [--bar:number] { if ($bar | is-empty) { echo "empty" } else { echo $bar } }; foo
         "#
     );